1. Field of the Invention
The present invention relates to a webbing drive apparatus for a seat belt system which is adapted to drive a webbing for restricting a seat occupant of a vehicle so as to automatically fasten the webbing to the occupant or cancel the application thereof.
2. Description of the Related Art:
An automatic seat belt system in which the webbing is driven by a webbing drive apparatus for automatically applying a webbing to a seat occupant or cancelling the application thereof after the occupant is seated in a vehicle has been proposed.
A conventionally known webbing driving apparatus is arranged in such a manner that the webbing is driven in the longitudinal direction of the vehicle by means of a wire. In this type of webbing drive apparatus, a drum is rotated by the driving force of a motor to take up the wire onto the drum or pay out the same from the drum. The webbing is then guided by a guide means disposed in the longitudinal direction of the vehicle and is driven in the same direction.
With this drive apparatus, the drum onto which the wire is taken up is completely covered with a cover. The wire is stored inside the cover. This cover is provided with a wire passing port through which the wire, that is being taken up onto the drum or paid out therefrom, passes. The wire is thus adapted to be paid out from the drum or taken up onto it through this passing port.
Formed at the edge of the wire passage port provided in the cover is a projecting portion which projects in the direction in which the wire is paid out. The arrangement is such that the wire is paid out from this projecting portion, and is inserted into a guide means disposed in the longitudinal direction of the vehicle.
However, with the above-described webbing drive apparatus, there is a drawback in that when the wire is paid out from the drum, the wire being paid out is intermittently brought into contact with an inner wall of the cover and generates noise, thereby imparting discomfort to the occupant.
That is to say, at the time when the wire is taken up spirally onto the drum and a portion of the wire in the outermost layer thereof located at one end of the drum is paid out in the paying-out direction through the passage port of the cover, that portion of the wire separates from the outer periphery of the drum and is then driven in sliding contact with the inner wall surface of the cover before it is paid out through the passage port. However, the portion of the wire paid out from inside the cover is subjected to a compressive load by means of sliding resistance or the like caused by the guiding means. The result is that the wire can buckle inside the cover, becoming deformed in the same direction as the axial direction of the drum. If the wire is further paid out in this state in which buckling has occurred, the wire moves impulsively in the direction opposite to the direction in which it has become deformed by buckling. In consequence, the wire is brought into contact with the inner wall surface, particularly the inner wall surface opposing the end surface of the drum, and this results in the so-called spring-back phenomenon occurs which generates noises. When this spring-back phenomenon occurs intermittently as the wire is further paid out, the resulting intermittent noises impart discomfort to the vehicle occupant.